Many modern roadway surfaces are composed of asphalt, macadam or other bituminous covering to form a smooth pavement. Over a period of time, weathering and extensive use of the roadways will render the pavement surface hard and brittle, resulting in irregularities and cracking in the pavement. Shrinkage cracks may develop allowing water penetration to cause further deterioration of the roadway surface. Once started, pavement deterioration increases at an accelerated rate and, unless prompt action is taken, complete reconstruction may be required. The breakdown of pavement structure may be further accelerated by the lack of proper drainage or the presence of corrosive foreign matter, such as chemicals or road salts. Improper construction, such as the use of too little or too much asphaltic binder, an unsatisfactory grade of aggregate or inadequate compaction may cause further pavement deterioration.
In response to these road pavement problems, reconditioning apparatus and equipment have been developed to rework, and, if necessary remove, the weathered pavement. A typical system utilizes heating apparatus to soften the pavement, followed by apparatus to rake or plane the surface. In some cases an additional coating is spread over the reworked surface and the mixture is compressed by compacting equipment.
A basic problem in restoring road surfaces has been heating the pavement surface to a temperature and depth sufficient to permit the surface material to be adequately worked. Intense heat must be applied in order to obtain sufficient heat penetration through the pavement surface because of the low heat conductivity of asphalt. However, intense heat cannot be applied for a long period since asphalt burns at a relatively low temperature. Such asphalt burning may result in sufficient damage to prevent reuse of the asphalt for resurfacing purposes. Furthermore, smoke from the burning asphalt pollutes the atmosphere.
To overcome this heating problem maintenance machines must frequently make multiple passes over the same section of roadway in order to heat and work the pavement to a sufficient depth. This procedure is inefficient and time-consuming and substantially increases maintenance costs. Other machines use multiple heater units each operating at a temperature below the asphalt burning point. A large number of such units are required to achieve the desired heat penetration, thus adding to the bulkiness and cost of the equipment.
Another basic problem in road maintenance is that the type of reconditioning needed varies greatly with different pavement sections depending upon the original composition, the extent of deterioration and other factors. For example, surfaces having bumps and irregularities as well as minor cracking may be sufficiently restored by heating and planing the surface. Asphalt planing may be the only practical method for treating roadway surfaces in tunnels and on bridges where a specified headroom clearance is required. Other roadways having deep ruts and grooves require more extensive working. It may be necessary to rake or scarify the heated pavement surface and then smooth and resurface the pavement. Moreover, some highway portions require special treatment, such as grooving to prevent automobile skidding.
The great variation in the type of machinery and process required to recondition different roadway sections necessitates the use of a variety of equipment, thus substantially increasing the cost of maintenance. Furthermore, multiple passes of such equipment are frequently needed in order to produce a finished surface of satisfactory smoothness and durability.